It is known to provide a horizontal spray bar on an agricultural vehicle and to supply liquid fertilizer or weed inhibitor to the spray bar under pressure for spreading the liquid medium over the ground as the vehicle progresses. Such a spray bar used for supply of the liquid to the nozzles may be termed a wet spray bar.
With many present day chemicals, it is necessary to control accurately the quantity of liquid applied to the ground per unit area and, to this end, it is necessary to use finely calibrated nozzles and to maintain a constant head of pressure so that a well-defined rate of flow can be obtained.
The pressure along the wet spray bar would decrease if the nozzles were fed directly from the interior of the spray bar and, to this end, it is known for an individual constant pressure flow control valve to be provided in the fluid coupling between the spray bar and each nozzle served by the bar. The provision of such constant pressure flow control valves ensures that a uniform pressure is applied to the nozzles and constant rates of flow can thus be obtained. Equally, however, if the nozzles are separately supplied with liquid under pressure, when mounted on a boom (dry spray bar), the provision of constant pressure flow control valves may nevertheless be desirable.